The present invention relates to a method for the measurement of dewpoint of gases in which method the measured gas is cooled with the help of cold gas down to the dewpoint, and the generated mist is detected optically.
The invention also concerns an apparatus applicable for the implementation of the method.
Significant energy savings are possible in industrial processes by means of optimizing the conditions of the drying process. Dewpoint is one of major parameters associated with these processes. The measured dewpoint can be, e.g., the dewpoint of exhaust air from the dryer hood of a paper machine or the acid dewpoint of flue gases from a coal-fired power plant.
Several different methods are currently available for the measurement of dewpoint. The most common type of optical dewpoint measurement devices might be the "chilled-mirror" detector, in which humidity precipitating on a cold, mirror-polished plane will cause a change in the optical measurement signal. This device is extremely accurate in clean conditions but noncompatible with the actual contaminating conditions in process use. The direct measurement of dewpoint can also employ change of capacitance or resistance, but these methods are hampered by the same contamination problems as the chilled mirror.
The indirect optical measurement of dewpoint is based on absorption in the UV or IR bands of the vapour. The measurement gives the absolute moisture content, from which the dewpoint can then be computed. Contamination and high process temperatures make the continuous use of these kinds of methods unreliable in industrial processes. Other methods available for absolute humidity measurement are based on the use of microwave and acoustic emissions. The former method is based on microwave absorption and/or change in the dielectric factor of air caused by water vapour. The microwave method is costly to implement while otherwise a feasible solution. A transducer based on acoustic emission is hampered by its large dimensions and the high costs of correlation techniques required.
Dewpoint can also be determined with the help of relative humidity measuring equipment if sensor temperature is known. Dewpoint then becomes a computational variable. Consequently, direct measurement of dewpoint gives a better alternative, since an indirect measurement device can output a computational dewpoint even if the transducer is defective.